The efficient dispersion of nanoparticles in matrix materials is becoming a critical aspect of many emerging technologies and the development of a general strategy for enhancing and maximizing the efficiency of dispersion will offer a significant advantage for a variety of academic and industrial applications. For example, the thermomechanical responses of polymers, which can provide limitations to their practical use, are favorably altered by the addition of trace amounts of nanoparticles. Similarly, addition of high refractive nanoparticles to polymeric materials can lead to a dramatic increase in the refractive index of the overall nanocomposite. In both cases, the level and utility of property enhancement is directly related to the degree of dispersion for the nanoparticle. Non uniform dispersion leading to nanoparticle aggregation is undesirable as it result in property degradation such as opacity for optical nanocomposites. Traditional approaches to nanoparticle dispersion involve the use of either small molecule ligands/surfactants or functionalized polymers such as block copolymers. However a number of challenges exist with these dispersing agents. For small molecule derivatives, low dispersion efficiency is often obtained due to the lack of entanglements and favorable interactions with the polymeric matrix. In contrast, polymeric dispersing agents can have favorable interactions and entanglements with the polymeric matrix, but the loading levels of these materials is often extremely high and is further exacerbated by the high surface area of nanoparticle systems. The weight percentage of the dispersing agent then becomes significant and leads to decreased performance.
A significant opportunity therefore exists to develop a general approach to the design of dispersing agents which combine the specificity and high binding strength of small molecules with the favorable interactions of polymeric dispersants. To address these issues, new dispersing agents were designed based on macromolecular architectures which optiminally present, both surface active groups for attaching to the surface of the nanoparticle, and matrix interacting groups which promote dispersion in the polymeric matrix. Hybrid dendritic linear block copolymers satisfy these criteria with the dendritic unit being used as the ‘head’ group to interact with the nanoparticle surface while the linear block is able to entangle and interact with the polymeric matrix. While dendritic macromolecules have found extensive use as stabilizing agents for nanoparticle formation, all of these studies have utilized the dendrimer as a nanoreactor for localized growth of the nanoparticles within the dendritic framework. No studies have been reported describing the use of hybrid dendritic linear block copolymers to stabilize the surface of nanoparticles even though the surface activity of these systems is well noted. In particular, the numerous reactive groups at the chain ends of the dendrimer have been shown to lead to an optimal conformation for interacting with surfaces. The absence of chain folding and chain dynamics when compared to functionalized linear chains is also expected to lead to a much stronger interaction with, and greater coverage of the nanoparticle at significantly lower loading of the block copolymer. Previously, Frechet has shown that poly(ethylene glycol) based hydrid structures can cover the surface of cellulose fibers at very low concentrations due to a combination of self-assembly and physisorption.